"I do not know any politicians."
Translation:Én nem ismerek politikusokat.
@Haroldnek: that would mean something like "Although I know some politicians, none of them are "not memorable" If you use "semmilyen", you describe the lack of their memorable qualities, but not deny that you know some politicians.
@Roberto343666: "semmit" means "nothing" in an accusative case, reference for inanimate objects and things. For persons you'd use "senkit", but that means "no one", therefore also cannot be used here.
This is why proper replies are important. The second part is correct, the first part, hmmm, very odd. I think your interpretation is rather forced and much less common than the interpretation the asker probably had in mind. You are interpreting "semmilyen politikus" as a characterless/featureless politician and putting it into a negative sentence - I think it's much more usual to interpret it as "not any kind" in a sentence like this. We do use "Nem ismerek semmilyen politikusokat" kind of sentences quite often to strongly emphasize the lack of the given part of speech - although I think it's more common for the noun to be in the singular.
tudni and ismerni are not interchangeable and, in fact, I think usually only one of them is appropriate at a time. tudni is to have knowledge or ability of something, like knowing how to swim or knowing where someone's house is. Whereas ismerni is to be familiar with somebody or something (usually people, but not always)
So, for example, maybe you "ismerem" your neighbor because you see him walking by and say hi every day, but you don't "tudom" his name; or you "ismerem" that restaurant that you ate at last night, but you don't "tudom" where exactly it is.